The present invention relates to an improvement in the structure of an electrophotographic photosensitive member, and more particularly to an electrophotographic photosensitive member for a laser beam printer using a diode laser. The present invention relates to an electrophotographic photosensitive member comprising a photoconductive layer made of amorphous silicon containing germanium incorporated thereinto, particularly to an electrophotographic photosensitive member comprising a conductive support and provided thereon in the following order, a barrier layer, a photoconductive layer, and a surface layer.
Description of Prior Art:
Amorphous selenium, a composite of cadmium sulfide (CdS) and an organic binder, an organic photoconductive member etc. has heretofore been used as an electrophotographic photosensitive member. A hydrogenated or halogenated amorphous silicon has recently attracted attention as a photoconductive materials for electrophotographic photosensitive member because of developing the preparation technique of high resistive film with high photoconductivity. This photoconductive material is believed to be a substantially ideal electrophotographic photosensitive member since it has not only a higher electrophotographic sensitivity than those of the conventional photoconductive materials but also a high hardness and a low toxicity.
Particularly the electrophotographic photosensitive member comprising a photoconductive layer made of amorphous silicon containing germanium, tin, or the like incorporated thereinto has a high sensitivity even at 750 to 820 nm which are oscillation wavelengths of the GaAlAs diode laser. Thus, some examples of such photosensitive materials are known as electrophotographic photosensitive members for a diode laser beam printer.
In, for example, Japanese Patent Laid-Open No. 192,044/1983, there is proposed which an electrophotographic photosensitive member having a structure as shown in FIG. 8. The conventional electrophotographic photosensitive member comprises a conductive support 101 and provided thereon in the following order, a high resistive film layer 102 (barrier layer or charge transport layer), a photoconductive layer 103 (charge generation layer), and a surface layer 104.
The high resistive film layer 102 (barrier layer or charge transport layer) is made of amorphous silicon containing carbon incorporated thereinto and has a dark resistivity of 10.sub.12 .OMEGA..multidot.cm or more. The photoconductive layer 103 (charge generation layer) is made of amorphous silicon containing germanium and has the sensitivity in the long wavelength range. The surface layer 104 is made of amorphous silicon containing carbon incorporated thereinto and has an optical gap of 2.3 eV or more. The surface layer 104 is transparent to visible light and infrared light.
In the above conventional electrophotographic photosensitive member, the photoconductive layer 103 disadvantageously has a lowered resistance because of incorporation of germanium therein and hence is poor in the charge acceptance. However, the charge acceptance is supplemented by additional provision of the high resistive film layer 102 and the high resistive surface layer 104, each made of amorphous silicon containing carbon, above and under the photoconductive layer 103, thereby improving electrophotographic characteristics such as dark decay and residual potential etc. Thus, an electrophotographic photosensitive member having the high sensitivity for the long wavelength light is provided.
The above conventional electrophotographic photosensitive member is thought to be one of those satisfying requirements of a photosensitive member having a high sensitivity for a long wavelength light, but is yet insufficient in many aspects. Specifically, the sensitivity peak of the electrophotographic photosensitive member is located at 700 nm, which is largely deviated from the oscillation wavelengths of the GaAlAs diode laser. When the sensitivity peak position is adjusted to approach to the oscillation wavelengths of the GaAlAs diode laser by increasing the amount of germanium, the maximum value of the sensitivity is disadvantageously decreased.
For explaining the above conventional electrophotographic photosensitive member, a model band diagram as shown in FIG. 9 is prepared on the assumption that optical gap of respective amorphous silicon layers are pseudo-band gaps (numerals in FIG. 9 correspond to those in FIG. 8).
The optical gap of the photoconductive layer 103 sensitive to a GaAlAs diode laser of 750 to 820 nm is thought to be 1.5 eV, while that of the surface layer 104 is 2.3 eV or more, thus providing a large energy difference. The interface between the photoconductive layer 103 and the surface layer 104 is a place where the layer of combination of silicon and germanium having substantially the same covalent bond radii is in contact with the layer of combination of silicon and carbon having largely different bond radii. In such a place, the localized state (interfacial state) density is high.
Such a large energy difference and a high interfacial state remarkably spoil the sensitivity of the electrophotographic photosensitive member. Specifically, charge carriers (holes or electrons) generated in the photoconductive layer 103 cannot reach the surface of the electrophotographic photosensitive member and the conductive support 101 because they cannot clear the large energy difference or are captured by the interfacial state and, therefore, cannot serve as an effective photoelectric current.
In particular, the photoconductive layer 103 made of amorphous silicon containing germanium has an additional problem that it can take charge of only a weak electric field among the electric fields which have been applied to the whole electrophotographic photosensitive member because of its resistance lower than those of the other layers, which leads to an increase in recombination efficiency of hole and electron through the above-mentioned process, thereby causing lowering in the sensitivity.
Furthermore in, for example, Japanese Patent Laid-Open No. 190,955/1983, it is proposed that an electrophotographic photosensitive member comprises a conductive support and provided thereon in the following order, a barrier layer, a charge transport layer, and a charge generation layer. The charge transfer layer is made of amorphous silicon or amorphous silicon containing boron. The charge generation layer is made of amorphous silicon containing germanium.
Further in this Japanese patent a surface protection layer may be added on the charge generation layer. The surface protection layer is made of amorphous silicon layer or amorphous silicon layer containing boron and provided thereon amorphous silicon carbide layer containing carbon.
The latter electrophotographic photosensitive member has electrophotographic sensitivity characteristics for a long wavelength light, however such electrophotographic sensitivity characteristics are yet insufficient for a longer wavelength light in various aspects.